Tien-Sung Tom Lin

We have employed both cw and pulsed EPR techniques to study the structure and dynamics of paramagnetic species: reactive intermediates, such as free radicals; photo-induced paramagnetism, such as photo-excited triplet state; transition metal containing compounds and enzymes. Below are two areas of our current interests.

Applications of zero-field (ZF) EPR spectroscopy. Recently, we developed a field and frequency agile pulsed EPR spectrometer to measure the properties and dynamics of photo-excited triplet state of organic molecules in ZF and low magnetic field. The conventional wisdom regards the first order hyperfine interaction (HFI) between electron and nuclear spin is zero in ZF because the magnetic moment is negligible in ZF. We have demonstrated the HFI can be observed in ZF during the pulsed EPR FID detection period for the photo-excited triplet state of organic molecules after the application of a short microwave pulse. The measurements in ZF and near ZF (0.1 - 1.0 mT) show that organic molecules remember the events happen shortly before \the measurements. The molecular memory and associated selective spin population in the photo-excitation could render organic solids a new class of materials for quantum computing. We have observed the drastic changes of spectral profile when we turn on a magnetic field as small as 0.4 mT by a field jump technique. We believe this is a promising experimental technique to examine a particular interaction, such as hyperfine, quadrupole, or other types of interactions by imposing a selective external field during the FID period. The technique allows us to examine the natural line width, spin-spin relaxation, dynamic nuclear polarization, and phase transition induced by molecular motions (Fig. 1).

Fig. 1 Left: Motion of the host (p-terphenyl) affects the guest (pentacene, center) orientation. Right: ZF spectral contour of pentacene triplet in p-terphenyl crystals as a function of temperature: (a) Protonated and (b) deuterated pentacene. Phase transition = 193 K.

Chemistry in the confined space of mesoporous silica (MPS) materials. We have studied the catalytic reactions and reaction pathways of enzymes and biomimetic model compounds immobilized in the nanochannels of MSP materials (pore diameter of 2 - 10 nm) by spectroscopic techniques: Uv-Visible, IR, EPR and NMR. These reactions are carried out in the nanopores (nanoreactors), which effectively make the concentration much higher and avoid the random attacks between fast-reacting species. Nanoreactors therefore open up the possibility of "single molecule chemistry" otherwise accessible only in extreme dilution. The nanopores of MPS provide proper environment, electrostatic attraction, geometry and configuration, to facilitate catalytic reactions, increase the stability of catalytic centers, and yield high turnover number (Fig. 2). Site-isolation of native enzymes and biomimetic complexes through encapsulation in porous solid materials further allow the use of less sterically demanding ligands while retains structural stability (Fig. 3).

Fig. 2 Enzymes immobilized in MPS.

Fig. 3 Site-isolation catalysis.

1985-present, Professor, Department of Chemistry, Washington University
1996-present, Professor of Neurology, School of Medicine, Washington University
1976-1985, Associate Professor, Department of Chemistry, Washington University
1970-1976, Assistant Professor, Department of Chemistry, Washington University
2006 May-June, 2005 March-April, Visiting Professor, Department of Chemistry, University of Frieburg, Germany
1997-2008 Summers, Visiting Professor, Department of Chemistry, National Taiwan University, Taiwan
1998, 1999, 2000 (16 months), Visiting Professor, Institute of Physical and Chemical Research, Japan (under Eminent Scientist Program)
1993-1996 Summers, Visiting Professor, Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan
1989-1990, Visiting Professor, Huygens Laboratory (Physics), University of Leiden, The Netherlands
1981-1981, Scientist-in-Residence, Chemistry Division, Argonne National Laboratory, Illinois